Wjs

, (autoxidation) -accelerated by heat

CH3(CH2)n-CH = CH — CH2_C_H —CH—_CH—_CH_—_CH —(CH2)nCOOH

(resonance) Free-radicals poty

O2 (peroxidation)

CH3CH2-CH = CH —CH2-CH —CH = CH —CH = CH —(CH2)nCOOH

(degradation)

short-chain compounds including aldehydes, ketones ^and short chain fatty acids

Figure 6 Autoxidation is caused by removal of a hydrogen from the methyl group between double bonds in polyunsaturated fatty acids. Resonating free radicals are produced and propagate peroxidation, degradation, and formation of polymers.

Polyunsaturated fatty acids with the methylene-interrupted double bond are also susceptible to oxidation (Figure 6). The hydrogen atoms on the methyl group between double bonds are susceptible to sequestration by oxidizing agents, such as iron or free radicals. This autoxidation results in a resonating free radical that is self-propagating and, with exposure to oxygen, yields peroxides. The peroxides may polymerize or degrade to smaller molecules. In foods, this process results in the condition of rancidity characterized by off flavors. In living systems, the products of peroxidation may cause reactions that damage proteins, membranes, and DNA resulting in pathological processes. Antioxidants are compounds that are capable of interrupting free radical propagation by reducing the peroxide to an alcohol without itself becoming a free radical. Tocopherols are a major antioxidant group in living systems and chemical antioxidants such as BHT (3,5-di-t-butyl-4-hydroxytoluene) are used in food products.

Another primary characteristic of naturally occurring PUFAs is that they cannot be synthesized by animals but are necessary for metabolism; therefore, they are an essential component of the diet. Animal organisms can introduce a double bond at the C-9 position but lack the enzymes to insert double bonds between the C-9 position and the methyl terminal carbon. The fatty acids are therefore considered to be in three families in relation to their biological functions: the mono-unsaturated (n-9 or omega-9) family and the polyunsaturated n-6 (omega-6) and n-3 (omega-3) families.

Phospholipids

The glycerol backbone is the central structure of phospholipids, as it is for acylglycerols. They are characterized by a phosphate group at the sn-3 position making phosphatidic acid (Figure 7).

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